IgE is a member of the immunoglobulin family that mediates allergic responses such as asthma, food allergies, type 1 hypersensitivity and the familiar sinus inflammation allergic rhinitis and conjunctivitis, and as a result, causes widespread suffering throughout the general population. IgE is secreted by, and expressed on the surface of, B-cells. IgE synthesized by B-cells can be anchored in the B-cell membrane by a short transmembrane domain linked to the mature IgE sequence. Membrane and secreted versions of IgE are formed in the same cell by differential splicing of the IgE RNA transcript.
IgE also can be bound to B-cells (and T cells, monocytes, Langerhans cells, follicular dendritic cells, natural killer cells, eosinophils and platelets) through its Fc region to a low affinity IgE receptor (FceRII, hereafter “FCEL”, and to mast cells and basophils through its Fc region to a high affinity IgE receptor FceRI, (hereinafter “FCEH”). The low affinity IgE receptor is generally referred to in the literature as CD23.
Upon exposure of a mammal to an allergen, antigen presenting cells process the antigen for presentation to helper T cells. These helper T cells secrete cytokines such as IL-4 which assist B-cells to undergo clonal amplification and secrete more allergen-specific IgE. This newly synthesized IgE in turn is released into the circulation where it binds to mast cells and basophils through the high affinity receptor on their cell surface. Such mast cells and basophils are thereby sensitized to the specific allergen. The next exposure to the same allergen causes binding to specific IgE on the surface of mast cells, and basophils, thereby cross-linking the FceRI on these cells and thus activating their release of histamine and other factors which are responsible for clinical hypersensitivity and anaphylaxis.
The art has reported antibodies capable of binding to FCEL (CD23)-bound IgE but not IgE bound to FCEH (see, for example, WO 89/00138 and U.S. Pat. No. 4,940,782). These antibodies are disclosed to be clinically advantageous because they bind to IgE which is bound to the low affinity receptor (FCEL) or to circulating IgE's, but do not bind to IgE bound to the high affinity receptor (FCEH). Therefore, these antibodies will not activate mast cells or basophils.
Moreover, anti-CD23 antibodies have been reported to have potential as therapeutics, e.g., for the treatment of allergic disorders, inflammatory diseases, and autoimmune diseases. For example, Bonnefoy et al., WO 9612741, report that ligands which bind CD23, e.g., monoclonal antibodies, are useful in the treatment or prophylaxis of inflammatory, autoimmune and allergic diseases.
The usage of monoclonal antibodies to CD23, as both IgE agonists and antagonists has been reported. IgE antagonists have been reported to have potential utility in treatment of conditions or diseases wherein IgE suppression is therapeutically desirable, e.g., allergic conditions such as allergic rhinitis and conjuntivitis, atopic dermatitis and asthma. For example, Bonnefoy et al., WO 8707302 (1987), report monoclonal antibodies to human CD23, which are assertedly useful for assaying the presence of IgE receptors on cell types and as therapeutics in diseases wherein modulation of IgE is therapeutically desirable.
In part because of their potential as therapeutics and diagnostics, many groups have reported the generation of monoclonal antibodies to CD23. See, e.g., Rector et al., Immunol., 55:481–488 (1985); Suemura et al., J. Immunol., 137:1214–1220 (1986); Noro et al., J. Immunol., 137:1258–1263 (1986); Bonnefoy et al., J. Immunol., 138:2970–2978 (1987); Flores-Romo et al., Science, 261:1038–1046 (1993); Sherr et al.; J. Immunol., 142:481–489 (1989); and Pene et al., Proc. Natl. Acad. Sci., USA, 85:6880–6884 (1988). Moreover, as discussed supra, the usage of such antibodies specifically to inhibit IgE production in systems where IgE synthesis is cytokine (IL-4) induced has also been reported. (Flores-Romo et al (Id.); Sherr et al. (Id.); Bonnefoy et al. (WO 8707302); Bonnefoy et al. (WO 8707302); Bonnefoy et al. (WO 9612741)); Bonnefoy et al., Eur. J. Immunol 20:139–144 (1990); Sarfati et al., J. Immunol 141:2195–2199 (1988) and Wakai et al., Hybridoma 12:25–43 (1993). Also, Flores-Romo et al. (Id.) disclose that Fabs prepared from anti-CD23 antibodies inhibit antigen-specific induced IgE responses in vivo in the rat. However, notwithstanding what has been reported, the mechanism by which anti-CD23 antibodies modulate IgE expression and in particular, the manner by which they block IL-4 induced IgE production remains unclear.
It has been suggested that anti-CD23 antibodies inhibit IgE production by signaling through CD23 present on the surface of IgE secreting B cells. It has been proposed that the function of CD23, which is upregulated on IgE secreting B cells, is feedback inhibition of IgE production (Yu, et al. Nature 369, 753–756 (1994)). This has been theorized because mice in which the CD23 gene has been removed have increased and sustained IgE production compared to controls (Yu, et al.). In addition, it has been reported that binding to CD23 by IgE complexes or by a monoclonal antibody to anti-CD23 suppresses ongoing IgE synthesis by a lymphoblastoid cell line that constitutively secretes IgE (Sherr et al. (Id.)). It appears that this is due to down regulation of the messenger RNA for the secreted IgE heavy chain in this cell (Saxon et al., J. Immunol., 147:4000–4006 (1991)). However, the exact mechanism by which IgE expression is inhibited has yet to be explained in systems in which IgE secretion is IL-4 induced.
It has also been reported that crosslinking of Fc gamma RII with surface Ig (B cell receptor) on B cells leads to down regulation of Ig expression. (D'Ambrosia et al., Science, 268:293–297 (1995). A similar mechanism can be proposed for B cells secreting IgE which also have cell surface CD23 and Fc gamma RII. An anti-human CD23 antibody bound to a cell by antigen (CD23) and also bound to Fc gamma RII through Fc interactions could transmit a signal to suppress IgE secretion through Fc gamma RII.
Mechanisms involved in IgE inhibition by anti-CD23 antibodies have been proposed that include blocking interactions other than the interaction between membrane CD23 and IgE. Related to this, CD23, which is a member of the C-type lectin family, has been shown to interact with several other ligands such as CD21, CD11b and CD11c present on a variety of cell types including T cells and monocytes. In this context CD23 can be envisioned as a cellular adhesion molecule.
Therefore, it has been proposed that the CD21–CD23 interaction may be involved in antigen presentation and subsequent IgE production. Models suggest CD21 on B cells sending an activation signal for IgE production after binding to CD23 on activated T cells present primarily in atopic individuals. (Lecoanet et al., Immunol., 88:35–39 (1996); and Bonnefoy et al., Int. Amer. Allergy Immunol., 107:40–42 (1995).) Blocking this interaction with an anti-CD23 could block induced IgE production. (Aubry et al., Nature, 358:505–507 and Immunol., 5:944–949 (1993); Grosjean et al., Curr. Opin. Eur. J. Immunol., 24:2982–2988 (1994); Henchoz-Lecoanet et al., Immunol., 88:35–39 (1996) Nambu et al., Immunol. Lett., 44:163–167 (1995); Bonnefoy et al., Int. Amer. Allergy Immunol., 107:40–42 (1995).) It is also possible that antigen presentation is upregulated by CD23 on antigen presenting B cells binding to CD21 on T cells.
Yet another mechanism which would potentially explain the effects of CD23 on IgE production involves soluble forms of CD23. It has been reported that CD23 is cleaved from the cell surface releasing several different forms of soluble CD23 or IgE binding factors. (Sarfati et al., Immunol., 53:197–205 (1984).) Soluble CD23 is a cytokine, with one of its reported activities being the augmentation of IL-4 induced IgE production from B cells. (Pene et al., J. Cell Biochem., 39:253–269 (1989); Pene et al., Eur. J. Immunol., 18:929–935 (1988); Sarfati et al., J. Immunol., 141:2195–2197 (1988); Sarfati et al. (1984) (Id.); (Saxon et al., J. Clin. Immunol. Allergy, 86 (3 pt 1) 333–344 (1990). Also, certain forms of soluble CD23 have been reported to inhibit IgE production (Sarfati et al., Immunol., 76:662–667 (1992)). Accordingly, anti-CD23 antibodies potentially may block IgE production by 1) inhibiting the IgE augmenting effects of soluble CD23 and/or 2) blocking the proteolytic release of soluble CD23 from the cell surface.
Thus, based on the foregoing, it is clear that there is significant complexity and uncertainty in the art with respect to the functions of more specifically CD23 and effects on IgE production, and further with respect to the means by which ligands specific thereto affect IgE production.